A safety device of this type is in German Published Patent Application No. 199 12 301 in which the impact detection system is formed by acceleration sensors which may simultaneously be used for activating an airbag system. If, in a collision with another vehicle, the acceleration measured by the acceleration sensor exceeds a certain threshold value and subsequently, toward the end of the collision, drops again below a certain threshold value, the vehicle brake is automatically activated after the drop below the last-mentioned threshold value. The intent is to minimize collision damage caused by an impact on the rear end of the vehicle in particular. The vehicle brake is still deactivated during the actual impact occurrence so that as large a part of the impact energy as possible is converted into kinetic energy, thereby making less energy available for the deformation of the host vehicle. Due to the automatic braking of the vehicle after impact, the danger is minimized of subsequent damage which may occur due to the fact that the host vehicle is propelled forward by the impact on the rear end and then in turn hits a vehicle in front.
On the other hand, for improving the safety and driving comfort, distance warning systems and distance control systems have been developed which have a position-finding system, in the form of a radar sensor, for example, which makes it possible to locate objects in front of the host vehicle and to measure their distances and relative speeds. If it turns out that the safety distance to a preceding vehicle is not sufficient then a warning signal is automatically output or the system automatically intervenes in the longitudinal guidance of the vehicle to regulate the distance to the preceding vehicle.
Automatic emergency braking systems have also been developed based on this technology. For example, German Published Patent Application No. 36 37 165 describes a system which, based on the positioning data of the radar sensor, automatically detects a situation in which a collision is presumably no longer avoidable and which calculates the likely time of collision and initiates countermeasures on this basis for reducing the collision damage. In the first step, only a warning indication is output to the driver. If the driver does not respond, a braking operation with moderate braking force is automatically initiated in a second step. If this is not sufficient to prevent the collision, then finally emergency braking with maximum braking force is automatically initiated. However, such emergency braking systems failed to become accepted in practice. A considerable disadvantage is the fact that the previously available sensor systems do not allow a reliable estimate of the traffic situation so that erroneous triggering may occur which in turn presents a considerable accident risk.
In view of this problem, German Published Patent Application No. 101 18 707 describes a system which does not automatically initiate an emergency braking operation when a collision risk is detected, but rather prepares the braking system for imminent emergency braking, e.g., by pre-loading the brake cylinders (pre-fill), so that, when the driver himself initiates emergency braking, the braking action may be deployed more quickly. However, avoiding the collision or alleviating collision damage is only possible in this system if the driver actively intervenes in the action.